Preheat the oven to 350°F (180°C). Grease and line a 6x3-inch round cake pan with parchment paper.
Whisk together the flour, cocoa powder, baking powder, baking soda, and salt in a bowl. Set aside. 83 g all-purpose flour, 25 g cocoa powder, 1/2 teaspoon baking powder, 1/4 teaspoon baking soda, 1/4 teaspoon salt
Measure the milk, add the vinegar, stir, and leave to sit for 2 minutes. 60 g whole milk, 1/2 teaspoon Apple cider vinegar
In a separate bowl, whisk together the egg, sugars, oil, and vanilla. 1 large egg, 66 g granulated sugar, 50 g soft brown sugar, 45 g oil, 1/2 teaspoon vanilla paste or extract
Add the dry ingredients to the wet in thirds, alternating with the milk mixture, folding gently with a spatula after each addition.
Pour in the hot coffee and fold until just combined. Do not overmix. 60 g hot coffee
Pour the batter into the prepared pan and bake for 30 to 35 minutes, until a skewer inserted in the center comes out clean.
Run a knife around the edge to loosen. Cool in the pan for 15 minutes, then turn out onto a wire rack and cool completely before frosting.